This webpage shows how you can build an f/stop
timer for B&W analogue printing.
If you just want to buy one,
both RHD
and DA (sold
out?) may
sell you a very nice working and well-supported unit. I will
not sell you a timer, but I usually have circuit boards for sale for those
who want to assemble their own. This is open hardware and software
so you can build one using the design files provided here; you don't
need to buy anything from me.

If you build this, please contact me (email address is in the
headers of all the source files) as I'd like to have some idea of
how many of these are out there. Of course if you have suggestions
or bug reports, please send them in too.

This is an enlarger timer. It controls (through duration of
exposure) how much light hits a piece of black and white photographic paper,
thereby controlling how dark it gets. By expressing exposures in a
log scale (stops, being log base 2), we can perform a lot of print
manipulations such as dodging and burning, changing size, shifting the image
by a fixed overall tone change, etc, just by performing simple
addition/subtraction arithmetic. And of course the timer does all
the arithmetic for you. It can also be used to make colour prints,
but it will do no colour computations for you.

While you can achieve the same ends with a linear timer, it's a lot
more taxing, both in the number crunching you have to do in your
head as well as needing to adjust the times for all exposures in a
print when making a global adjustment, e.g. for print
magnification. All of that is taken care of by an f/stop timer,
including the ability to program complex (up to 8 steps) sequences
of dodge/burn actions into the timer.

The term "f/stop timer" was coined by Gene Nocon, who is widely
credited with inventing this approach. Nocon sold an f/stop timer
and more importantly, called it an f/stop timer - even though the
process has NOTHING to do with apertures except the logarithmic
spacing of the values used.

Here's a
brief article
explaining what f/stop timing is all about and here's
a primer
from RHD (the manufacturer of another of these timers).

Serial communications implemented in order to save/load
programs... however there is not yet a matching PC software to talk
to the timer

Using the Focus button no longer resets the current program, so
you can do a focus-exposure in the middle of a program.
Split-grade printing on a colour enlarger with backlit wheels (ike
mine!) was a real hassle prior to this fix.

Rotary encoder direction can be reversed

Code updated to work under Arduino 1.0 instead of 0022.

Bugfix wrt saving to 7th slot (anything you saved under v0.3 in slot N
will now appear in slot N+1; slot 1 will contain invalid settings)

Features "coming soon" (for values of
soon that include I-hope-someone-else-sends-me-a-software-patch):

Host software to manage a library of print programs on a
Windows/Linux/OSX PC and transfer them to/from the timer

The "not implemented" features have
placeholders in the code and they may appear in the next few
years. Missing features that are likely to stay missing for the
foreseeable future are: